The invention relates to self-heating devices for providing heating by exothermic chemical reactions.
Compact, self-heating devices that produce heat through exothermic chemical reactions are known to the art. U.S. Pat. No. 4,397,315 discloses a device having an outer envelope and an inner envelope, with the outer envelope containing sodium thiosulfate, and the inner envelope containing ethylene glycol. The walls of the inner envelope are rupturable, allowing the contents of each envelope to mix. U.S. Pat. No. 5,035,230 discloses a heat pack having two compartments separated by a frangible seal. Potassium permanganate oxidizing agent coated with sodium silicate is provided in one zone of the heat pack, and aqueous ethylene glycol fuel is provided in the other zone. In operation of the device, the seal is compromised to allow the reactants to come in contact with each other.
Devices for producing heat or cold by heat of Hdilution rather than by chemical reaction are also known. U.S. Pat. No. 3,804,077 describes a heat pack which contains calcium chloride, a water soluble chemical and starch, a gelling agent, in one zone, and water in another zone.
A characteristic feature of a heat pack is the attainment of an operating temperature as measured on the surface of the heat pack. The operating temperature depends on the balance between the heating rate (heat generation rate) and the cooling rate (heat transfer to the surroundings). In the best heat packs according to the prior art, the presence or absence of a heat absorbing body external to the heat pack and the rate of heat loss did not affect the progress of the exothermic chemical reaction. Thus, a heat pack according to the prior art achieved a stable balance between heat generation within the pack and heat transfer to the pack's surroundings only for an intended, or design, rate of heat loss. The design rate of heat loss does not occur in every instance. In particular, if heat transfer to the surroundings is less than intended by design (a poorly cooled pack), the rate of heat generation in the pack and the consequent temperature rise are relatively excessive in that instance. Situations in which heat loss below design can occur are high ambient temperature or low thermal mass in the heat sink (low .DELTA.T). Another situation is poor physical contact to a heat sink (low heat transfer coefficient). When poorly cooled, packs of the prior art are prone to produce temperatures higher than intended and excessive buildup of pressure (e.g., vapor pressure, such as from steam) can cause the pack to rupture, sometimes explosively.